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Micronutrient deficiencies threaten global food production. Attempts to biofortify crops rely on a clear understanding of micronutrient uptake processes. Zinc deficiency in rice is a serious problem. One of the pathways proposed for the transfer of zinc from soils into rice plants involves deoxymugineic acid (DMA), a phytosiderophore. The idea that phytosiderophores play a wider role in nutrition of Poaceae beyond iron is well established. However, key mechanistic details of the DMA-assisted zinc uptake pathway in rice remain uncertain. In particular, questions surround the form in which zinc from DMA is taken up [i.e. as free aqueous Zn(II) or as Zn(II)–DMA complexes] and the role of competitive behaviour of other metals with DMA. We propose that an accurate description of the effect of changes in pH, ligand concentration, and ionic strength on the stability of Zn(II)–DMA complexes in the presence of other metals in the microenvironment around root cells is critical for understanding the modus operandi of DMA during zinc uptake. To that end, we reveal the importance of geochemical changes in the microenvironment around root cells and demonstrate the effect of inaccurate stability constants on speciation models. [ABSTRACT FROM AUTHOR]

Premise: In patterned fens, patches of short, sparse, species‐rich vegetation often occur on substrates rich in precipitated carbonates near calcareous springheads, with taller, denser vegetation farther away. Boyer and Wheeler (1989) hypothesized that phosphorus co‐precipitation near springheads limits plant productivity and coverage, and Givnish (1982) proposed that aggregations of rare, short‐statured plant species might reflect their competitive restriction to sparsely covered microsites. Methods: We tested these hypotheses by quantifying species distributions, leaf heights, plant coverage, community composition, and substrate and leaf chemistry of Eupatorium perfoliatum along a gradient of hydrology and geochemistry in a wetland complex in southeastern Wisconsin, USA, ranging from marl flats and fens on peat mounds near springheads to surrounding sedge meadows. Results: Community composition was strongly correlated with a one‐dimensional environmental gradient along which coverage and height increased moving downslope from marl flats, while soil carbonate, phosphorus immobilization capacity, and local species richness decreased, consistent with theory. Regionally rare species were short and restricted to sparsely covered microsites; within and among species, leaf height increased with local coverage. NPK tissue stoichiometry did not entirely support the Boyer‐Wheeler hypothesis, although nitrogen limitation appeared strongest in sedge meadows. Shifts in stature and tissue chemistry of E. perfoliatum along the marl flat–sedge meadow gradient suggested that zinc toxicity may help limit coverage near springheads despite no significant change in soil zinc content. Conclusions: We propose a modified Boyer‐Wheeler hypothesis to account for cascading effects of phosphorus co‐precipitation near springheads on nitrogen fixation, nitrogen+phosphorus co‐limitation, and zinc mobility. [ABSTRACT FROM AUTHOR]

Searching for life when we do not know what it is like is at best a difficult task, and at worst, an exercise fraught with frustration and potential failure. In developing methods that will have general applicability, one must ascertain first what the general features of life might be that, when present, would provide strong inference of the presence of active life. We propose here that the tendency of life to be associated with energetic gradients, in fact to create its own gradients via the catalytic consumption of nutrients and oxidants, and the production of waste products, is one of the general features of active extant life. As life seeks out and creates such gradients, layered microbial communities, or LMCs, result that are in their own way, prima facie biosignatures. These LMCs are abundant, perhaps ubiquitous in physically stratified environments on our own planet. If energy flows through a given system, and if life has developed efficient catalytic methods to utilize this energy, then LMCs are predicted to be present. Here we discuss the general issues of what can be gleaned from the study of LMCs in terms of energy flow, carbon flow, and carbon storage in earthly ecosystems, as well as the philosophy of using LMCs as biosignatures in the search for life on Earth, and perhaps, in extraterrestrial sites. [ABSTRACT FROM AUTHOR]